Table 1.

Morphological, esp. histopathological criteria used for classification

	Conventional schwannomaa	Melanotic schwannomaa,b	Leptomeningeal melanocytoma (MC)c	Cellular blue nevusb
Localization	Majority outside central nervous system, most often skin and subcutaneous tissue	Near the neural axis (esp. spinal nerve roots, cranial nerves) or peripherally located in a wide variety of sites (soft tissue, skin, etc.)	Mostly at the cervical and thoracic spinal level, sometimes posterior fossa and supratentorial compartment	Skin (mainly dermis); frequently buttocks or sacrococcygeal region
Origin	Nerve sheath (schwann cell)	Nerve sheath (schwann cell)	Leptomeningeal melanocyte	Dermal melanocyte
Growth pattern, margin	Circumscribed, encapsulated	Circumscribed, often encapsulated	Circumscribed, non-encapsulated; MC with invasion of CNS is classified as intermediate-grade MC	Non-encapsulated; both infiltrative and pushing border
Melanin pigmentation	Absent	Variably, often heavily pigmented	Variably, often heavily pigmented; can rarely be absent	Variably; can rarely be absent
Psammoma bodies and/or fat	Absent	Present in the psammomatous form	Absent	Absent
Schwannian featuresd	Present	Often less pronounced than in conventional schwannoma	Absent	Degenerative changes reminiscent of ancient schwannoma can be present
Cell phenotype	Spindle	Spindle, epithelioid	Spindle, epithelioid	Spindle, ovoid, dendritic (biphasic architecture)
Nuclear atypia	Generally absent; ancient changes (‘degenerative atypia’) can be present	Usually mild; prominent nuclear atypia with increased mitotic activity and necrosis is indicative of aggressive behavior	Generally absent; MC with increased mitotic activity (2–5 per 10 HPF) is classified as intermediate-grade MC	Variable; absence of necrosis, mitotic activity < 2 per 2 mm2
S-100	Positive	Positive	Positive	Positive
HMB-45 and/or Melan-A	Negative	Positive	Positive	Positive
Basement membrane staining	Pericellular pattern	Pericellular or biphasic (pericellular and nested) pattern	Nested pattern	Predominantly nested pattern

^aBased on WHO criteria [24]

^bBased on criteria described by Mooi [19, 20]

^cBased on criteria described by Brat et al. [1, 2]

^dEsp. Antoni A and B pattern, Verocay bodies and lipid-laden macrophages